As a pilot I can say this. While operating under IFR Regulations you must be able to fly to your destination with a 45 minute reserve. Now if you have to have an alternate, you must carry enough fuel to fly to your destination, then your alternate at a normal cruise speed and land with a 45 minute reserve. All airlines carry extra fuel reserves but there is a point when you will need to declare "min fuel" which means that any further delay could push you into your fuel reserves. The NWA crew probably declared min fuel at that time because they knew that they were going to have to divert and they wanted to divert and still have their reserve. This is more common than one would think. An airplane doesnt have an infinate number of fuel, at somepoint you are going to have to get more gas.